Tin-plated steel sheets and electrolytically chromated steel sheets referred to as tin-free steel sheets are used for various metal cans such as beverage cans, food cans, pail cans, and 18-litter cans. In particular, the tin-free steel sheets are produced by electrolyzing steel sheets in a plating bath containing hexavalent chromium and have the advantage of having excellent wet resin adhesion to resin such as paint.
In recent years, the use of hexavalent chromium has tended to be restricted worldwide in response to growing environmental awareness. Alternatives to the tin-free steel sheets, which are produced using such a plating bath containing hexavalent chromium, have been needed. For example, Japanese Unexamined Patent Application Publication No. 2004-285380 discloses a steel sheet, electrolyzed in a tungstic acid solution, for containers. Japanese Unexamined Patent Application Publication No. 2001-220685 discloses a surface-treated steel sheet, coated with a phosphonate layer, for containers. Japanese Unexamined Patent Application Publication No. 2002-355921 discloses a steel sheet, coated with a surface treatment layer containing one or both of Sn and Ni, for containers, the surface treatment layer being overlaid with a resin layer which contains one or both of tannic acid and acetic acid and Ti, Zn, or one or more of compounds thereof and which has a phenol structure. Japanese Unexamined Patent Application Publication No. 2006-009046 discloses a surface-treated metal material having an organic surface treatment layer and an inorganic surface treatment layer which contains no phosphate ion but principally contains T, O, and/or F.
Various types of metal cans have been conventionally manufactured in such a manner that metal sheets such as tin-free steel sheets are painted and then formed into can bodies. In recent years, the following method has been widely used to reduce the amount of industrial waste: a method in which a resin-coated metal sheet that is not painted but is coated with resin such as a resin film is formed into a can body. For the resin-coated metal sheet, the resin needs to strongly adhere to the metal sheet. In particular, resin-coated metal sheets used for beverage or food cans need to have high wet resin adhesion such that no resin is peeled therefrom even in high-temperature humid environments, because the beverage or food cans are subjected to retort sterilization steps in some cases after contents are packed in the beverage or food cans.
Furthermore, the resin-coated metal sheets need to have high corrosion resistance such that the cans are prevented from being perforated by corrosion due to the contents of the cans even if resin is partly peeled from the cans by scratching.
The following sheets and material have insufficient wet resin adhesion when being used in a retort atmosphere: the steel sheet electrolyzed in the tungstic acid solution as disclosed in Japanese Unexamined Patent Application Publication No. 2004-285380, the surface-treated steel sheet coated with the phosphonate layer as disclosed in Japanese Unexamined Patent Application Publication No. 2001-220685, the steel sheet having the resin layer having the phenol structure as disclosed in Japanese Unexamined Patent Application Publication No. 2002-355921, and the surface-treated metal material having the organic surface treatment layer and the inorganic surface treatment layer principally containing T, O, and/or F as disclosed in Japanese Unexamined Patent Application Publication No. 2006-009046.
It is an object of the present invention to provide a surface-treated steel sheet which contains no Cr, which has excellent wet resin adhesion, and which can be used as an alternative to a tin-free steel sheet; a method for producing the surface-treated steel sheet; and a resin-coated steel sheet produced by coating the surface-treated steel sheet with resin.